Various types of magnetic field sensing elements are known, including Hall Effect elements and magnetoresistance elements. Magnetic field sensors generally include a magnetic field sensing element and other electronic components. Some magnetic field sensors also include a fixed permanent magnet.
Magnetic field sensors provide an electrical signal representative of a sensed magnetic field. In some embodiments, the magnetic field sensor provides information about a sensed ferromagnetic object by sensing fluctuations of the magnetic field associated with the magnet part of the magnetic field sensor as an object moves within a magnetic field generated by the magnet. In the presence of a moving ferromagnetic object, the magnetic field signal sensed by the magnetic field sensor varies in accordance with a shape or profile of the moving ferromagnetic object.
In other embodiments, the magnetic field sensor has no magnet, and the magnetic field sensor provides information about a sensed object to which a magnet is coupled.
Magnetic field sensors are often used to detect movement of features of a ferromagnetic gear, such as gear teeth and/or gear slots. A magnetic field sensor in this application is commonly referred to as a “gear tooth” sensor.
In some arrangements, the gear is placed upon a target object, for example, a camshaft in an engine, thus, it is the rotation of the target object (e.g., camshaft) that is sensed by detection of the moving features of the gear. Gear tooth sensors are used, for example, in automotive applications to provide information to an engine control processor for ignition timing control, fuel management, and other operations.
In other embodiments, a ring magnet with a plurality of alternating poles, which can be ferromagnetic or otherwise magnetic, is coupled to the target object. In these embodiments, the magnetic field sensor senses rotation of the ring magnet and the target object to which it is coupled.
Information provided by the gear tooth sensor to the engine control processor can include, but is not limited to, an absolute angle of rotation of a target object (e.g., a camshaft) as it rotates, a speed of rotation, and, in some embodiments, a direction of rotation. With this information, the engine control processor can adjust the timing of firing of the ignition system and the timing of fuel injection by the fuel injection system.
Gear tooth sensors can include internal “detectors” that fall into two categories, namely, true power on state (TPOS) detectors, and precision rotation detectors. The two categories are generally distinguished by two characteristics: the speed with which they can identify edges of a gear after they are powered up, and the ultimate accuracy of their ability to detect the edges of the gear and place edges of an output signal at the proper times. TPOS sensors tend to be fast but have lower accuracy, while precision rotation detectors tend to be slower to detect a tooth or valley, but have higher accuracy.
Precision rotation detectors tend not to provide an accurate output signal (e.g., indication of tooth or valley) immediately upon movement of the target object from zero rotating speed, but instead provide an accurate output signal only once the target object has moved through at least one tooth/valley (pitch) of the target. For example, in one type of magnetic field sensor described in U.S. Pat. No. 6,525,531, issued Feb. 25, 2003, a positive digital-to-analog converter (PDAC) and a negative digital-to-analog converter (NDAC) track positive and negative peaks of magnetic field signal, respectively, for use in generating a threshold signal. A varying magnetic field signal is compared to the threshold signal. However, the outputs of the PDAC and the NDAC may not be accurate indications of the positive and negative peaks of the magnetic field signal until several cycles of the signal (i.e., signal peaks) occur (i.e., until several gear teeth have passed).
In contrast, a true power on state (TPOS) detector can provide a moderately accurate output signal state (e.g., indication of tooth or valley) before movement of a target object (e.g., camshaft) from zero rotating speed. Furthermore, even when the target object is not moving, the TPOS detector can provide an indication of whether the TPOS detector is in front of a gear tooth or a valley. The TPOS detector can be used in conjunction with a precision rotation detector in a common integrated circuit assembly, both providing information to the engine control processor at different times.
As described above, the conventional TPOS detector provides an accurate output signal before rotation of the target object and before the precision rotation detector can provide an accurate output signal. The TPOS detector can provide information to the engine control processor that can be more accurate than information provided by the precision rotation detector for time periods at the beginning of rotation of the target object, but which may be less accurate when the object is rotating at speed.
For embodiments that include both a TPOS detector and a precision rotation detector in a common integrated circuit assembly, when the object is rotating at speed, the engine control processor can primarily use rotation information provided by the precision rotation detector. In most conventional applications, once the magnetic field sensor switches to use the precision rotation detector, it does not return to use the TPOS detector until the target object stops rotating or nearly stops rotating.
A conventional TPOS detector is described in U.S. Pat. No. 7,362,094, issued Apr. 22, 2008. The conventional TPOS detector includes a comparator for comparing the magnetic field signal to a fixed, often trimmed, threshold signal. The conventional TPOS detector can be used in conjunction with and can detect rotational information about a TPOS cam (like a gear), which is disposed upon a target object, e.g., an engine camshaft, configured to rotate.
An output signal from a conventional TPOS detector has at least two states, and typically a high and a low state. The state of the conventional TPOS output signal is high when over one feature (e.g. tooth) and low when over the another feature (e.g. valley) as the target object rotates, in accordance with features on the TPOS cam attached to the target object. Similarly, the output signal from a conventional precision rotation detector has at least two states, and typically a high and a low state.
Gear tooth sensors depend upon a variety of mechanical characteristics in order to provide accuracy. For example, the gear tooth sensor must be placed close to (i.e., at a small air gap relative to) the ferromagnetic gear, teeth and valleys of which it senses as they pass. A larger air gap results in a smaller signal processed by the gear tooth sensors, which can result in noise or jitter in positions of edges of the two-state output signal generated by the gear tooth sensor. In radial sensing configurations, radial asymmetry of the ferromagnetic gear sensed by the gear tooth sensor can result in an air gap that varies as the ferromagnetic gear rotates. The radial asymmetry can take a variety of forms. For example, the radial asymmetry can result from an axis of rotation not centered in the ferromagnetic gear or from bent or missing gear teeth. In addition, wobble of the ferromagnetic gear can result in an air gap that varies. Wobble is described more fully below in conjunction with FIG. 2.
Radial asymmetry and wobble can be the result of circumstances that happen during production of an assembly, for example an engine. Dropping an assembly during production can bend a gear sensed by the gear tooth sensor.
A gear tooth sensor can generate what appears to be a proper output signal even when exposed to a gear with radial asymmetry or with wobble. However, the gear tooth sensor can be in a marginal condition subject to failure if any further variation in the air gap occurs, for example, due to temperature changes.
Thus, it would be desirable to provide an arrangement that can sense not only a pass/fail condition, but also a marginal condition in an assembly that contains a gear tooth sensor and a sensed object (e.g. a ferromagnetic gear).